1. Field of the Invention
The present invention relates to cooking devices; specifically, it relates to a system and method for AC line voltage analysis.
2. Description of the Related Art
Variations in the AC line voltage supplied to restaurant food service equipment normally degrade cooking performance. These variations may result in incomplete cooking, overcooking, unappealing appearance, and substandard taste. Thus, this equipment may be equipped with devices that measure AC line voltage to detect such variations.
Referring to FIGS. 1-3, schematics of known devices to measure AC line voltage are provided. In FIG. 1, device 100 includes resistors 102 and 104 that lower a high line voltage to a low voltage that is safe for input to A/D converter 112. Bridge rectifier 106 half-wave rectifies the AC waveform. Resistor 108 and capacitor 110 filter the half-wave rectified waveform to a DC voltage. This signal is input to A/D converter 112, and the digital information is processed by microprocessor 114.
Although it is a relatively simple device, device 100 has its drawbacks. Specifically, device 100 does not have line isolation, which makes agency approvals, such as those from Underwriter""s Laboratories, Inc. (xe2x80x9cUL(copyright)xe2x80x9d), difficult to obtain. In addition, because five distinct components are required, device 100 is costly.
Referring to FIG. 2, a second device for measuring AC line voltage is provided. Device 200 includes dedicated step-down transformer 202 that drops a high line voltage to a lower voltage. Bridge rectifier 204 full-wave rectifies the AC waveform. Resistor 206 and capacitor 208 filter the output of bridge rectifier 204 to a constant DC voltage, which is input to A/D converter 210, and processed by microprocessor 212.
Device 200 provides the line isolation that device 100 failed to provide. Transformer 202, however, is very large, heavy, and expensive. Further, the transfer function for transformer 202 is not tightly specified.
Referring to FIG. 3, a third known device for measuring AC line voltage is provided. Device 300 includes resistor 302, which limits the current input to optical isolator 304. Output 306 is a rectified AC waveform. Resistor 308 and capacitor 310 filter this waveform. Signal conditioning circuit 312 provides gain and offset to present a correct and suitable voltage to A/D converter 314, the output of which is processed by microprocessor 316.
Device 300 provides optical isolation from the line voltage. Resistor 302, however, must be a high-power resistor, which is inherently large and generates heat. Optical isolator 304 has transfer characteristics that are poorly controlled and change over time. Finally, signal conditioning circuit 312 adds an additional part and increases overall manufacturing costs.
Therefore, a need has arisen for a system and method for AC line voltage analysis that overcomes these and other deficiencies in the related art.
According to one embodiment of the present invention, a system for AC line voltage analysis is disclosed. The system for AC line voltage analysis on an AC line includes a transformer that steps down the AC line voltage, a rectifier that rectifies the stepped down AC line current, a filter that filters the rectified AC line current, a voltage divider that reduces the AC line voltage; and an A/D converter that converts the reduced AC line voltage to digital bits. According to another embodiment of the present invention, a system for AC line voltage analysis on an AC line includes a device that calibrates equipment for measuring AC line voltage analysis, a device that adjusts for at least one load, a device that filters an adjusted measurement, and a device that determines an actual line voltage.
According to another embodiment of the present invention, a method for AC line voltage analysis is disclosed. The method for AC line voltage analysis of an AC line includes the steps of calibrating equipment for measuring AC line voltage analysis, adjusting for at least one load, filtering an adjusted measurement; and determining an actual line voltage.
The method determines the loading effect of loads, such as the heat relay and the pressure solenoid, as well as LED light bars and alphanumeric displays. It may not be necessary to determine the loading effect for all loads.
The net result of the first calibration step is a table of offsets, in A/D bits, which are applied during subsequent AC voltage measurement.
Two known, constant AC line voltages are applied to the transformer, and the process control measures the A/D bit outputs at these two voltages and performs a two-point calibration. The resulting equation is then later used during normal operation to calculate the AC line voltage from the A/D measurement.
After the calibration is performed, the AC line voltage measurement is carried out by the process control. The control software reads the A/D converter.
The control software modifies the A/D reading by the appropriate offsets for load which were one during the A/D measurement.
The control software calculates the AC line voltage from the transfer function found during calibration.
The control software converts the AC line voltage measurement to a percent of nominal line voltage, for the purposes of comparison and indication. This is a user-interface convenience in that the end user need not know the nominal line voltage: if the control reports a line voltage of 100%, the user knows that the line voltage is correct, regardless of the actual nominal line voltage.